Over the past decade, we have witnessed tremendous progress in our understanding of the role of RNA molecules in the regulation of gene expression. The main contribution to this progress was offered by the discovery of RNA interference (RNAI). First identified in C. elegans by Fire and Mello, RNAi is an evolutionary conserved mechanism that brings about a sequence specific, post transcriptional gene silencing (PTGS) through the use of short RNAs. RNA interference (RNAi) is a short interfering RNA (siRNA) mediated gene siliencing technology with specific sequence. siRNA is duplex small molecule consisting of of 21-23 nucleotides. Because RNAi has efficient and specific effect on gene silencing to disease, it has been gradually developed to a new gene treatment method for inherited or acquired diseases which includes viral infections and cancer, and so on. Till now, a lot of animal treatment experiments and clinical trials have been carried out on using siRNA to treat disease.
The major limitations for the use of siRNA both in vitro and in vivo are the instability of naked siRNA in physiological conditions, rapid clearance from the bloodstream, and the inability to cross the cellular membrane to gain access to the intracellular environment. Although siRNA has shown good silencing effect on in vitro application research and the cell level, because of its large molecular weight and a large number of negative charges carried by itself, it is not able to penetrate through the cell membrane and enter into the cell, non-unique target losing effects and immune response will be initiated in the transportation. In the meanwhile, it also faces some other obstacles, such as nuclease degradation and so on, which results tremendous challenges in diseases treatment for siRNA transport system. So the transfection of siRNA has become the main bottleneck and restricted its application. How to enhance the ability of siRNA to penetrate the cell membrane and improve its stability and targeting in vivo are the urgent problems to be solved in siRNA drug carrier. So safe and effective siRNA drug carrier design and synthetise has become an important direction on siRNA drug research and development. Chemical modifications in the sugars, nucleobases, and the phosphate ester backbone of siRNA have been applied to improve its nuclease resistance without interfering with the silencing efficiency. Conjugation with hydrophobic functional groups has also enhanced the cellular uptake.
In comparison with chemical modifications of NAs, carrier-mediated strategies are emerging as a simple and fast means to formulate NA therapeutics and protect them from degradation. The carriers, including viral vectors and non-viral vectors, co-assembled or covalently conjugated with siRNA, The carriers are designed to enhance cell targeting, prolong drug circulation time, and improve membrane permeation. Because of the potential safety problems and high cost, wide range of clinical applications o viral vectors are greatly limited. Common delivery carrier of non-viral gene drug is positively charged cationic compound, it includes polycation, positively charged phospholipids, chitosan, albumin, dendritic macromolecules and peptide, etc., they compress the gene to assembled particles by the positively charged groups and the negatively charged phosphate groups on DNA/RNA, and make gene smoothly pass through all kinds of obstacles to complete the gene transfer, such as immune system escape, cell membrane penetration, endosome release, and so on. Although there are some arguments on what is a clear relationship between the construction and properties and transfection efficiency of DNA/RNA complexes there are still a lot of conditions determining the biocompatibility and the efficiency of the transfer DNA/RNA of the complexes according to experience. RNA/DNA carrier system formed by simple mixture and only electrostatic interactions is not stable, so a more stable and suitable sized carrier system is prerequisite to be designed for gene drugs to have good effect the of gene drugs.
As a kind of biological molecules, peptides have a lot of advantages to be a gene carrier that synthetic polymers does not have. Peptides have 20 kinds of amino acids that have different characteristics to form a primary sequence with tremendous properties; Beta folding, alpha helix and other secondary structure is obtainable by sequence design. Molecular with high purity, simple distribution and clear structure is obtainable by solid-phase synthesis. It is easy to be modified or connected to cellular target sequence, and enhance specificy. Peptides have been employed to deliver synthetic drugs, small molecules, bioactive peptides, therapeutic proteins, and NAs by a mechanism that has not yet been fully understood. These peptides may include protein-derived cell penetrating peptides (CPPs), cationic peptides, designed amphiphilic peptides, fusogenic peptides, cell targeting peptides (CTP) and peptides containing a nuclear localization signal. Cationic peptides rich in basic amino acids can electrostatically interact with small NAs or condense NA into small stable particles. CPPs can facilitate the translocation of the complex through the cell membrane. Histidine-rich pH-sensitive or fusogenic peptides can enhance the endosomal escape and cytoplasmic release of the gene complex. Involvement of CTPs in gene delivery systems mediates cell and/or tissue-specific targeting. Finally, attachment of a NLS peptide improves nuclear localization of the gene complex.
Although researches on siRNA drug delivery carrier has made some breakthroughs and developments, there are still some key issues need to be resolved to make siRNA drugs successfully been used in treatment of human diseases. For example, toxicity, specificity, targeting, immune stimulation, low transfection efficiency, and so on. Thus, because biodegradable and bicharacteristic peptide carrier can effectively combine and protect siRNA molecules to enhance cellular uptake of siRNA molecules, and improve its stability and targeting, and effectively release siRNA molecules, and reach the purpose of treatment, it makes such carrier has better advantage and application prospect in siRNA drug delivery.